Jenny, you can start off doing plenty through PADI. Here's a list of applicable specialties:
Project AWARE
AWARE - Fish Identification
Underwater Naturalist
National Geographic Diver
Coral Reef Conservation
Those would get you a quick and relatively inexpensive introduction.
Additionally, you might find this site helpful as well:
Biology Learning Center -Online Science Courses. For something more in depth, if you have the time, you could do a semester at sea program like
Sea|mester - Global semester at sea programs for college students with Marine Biology & Oceanography or simply audit courses at a university if there is one near you that offers that (I don't know how the universities in England operate, but here in the States you can often pay a nominal fee to sit in on a course of interest).
Hope this helps.
Jason